Image compensation method and apparatus for electrophotographic devices

ABSTRACT

A compensation method and apparatus wherein an electrometer probe is positioned in proximity to an exposed area of a charged photoreceptor surface. The probe is positioned only in that position of the area which is always light struck. Measurement of the surface potential and comparison of the surface potential to a fixed reference provides a control signal which can be employed to compensate charge, exposure, transfer and development.

O Unlted States Patent 1191 1111 3,788,739 Coriale Jan. 29, 1974 [54] IMAGE COMPENSATION METHOD AND 3,604,925 9/1971 Snelling et a], 355 3 x APPARATUS FOR 3,620,614 11/1971 Gunto et al 355/3 'Volkers [75] Inventor: Samuel D. Coriale, Webster, NY. p E p d L Braun [73] Assignee: Xerox Corporation, Rochester, NY, Attorney Agent, or irm jame5 Ralabate et [22] Flledz June 21, 1972 [57] ABSTRACT [21] Appl A compensation method and apparatus wherein an electrometer probe is positioned in proximity to an ex- 52 us. c1 355/17, 96/1 R, 355/3 posed area of a charged photoreceptor surfaoo- The [51] Int. Cl G03 13/00 Probe is Positioned only in that Position of the area [53] Fi ld f Search n 3 55 3 13 1 7; 9 R which is always light struck. Measurement of the surface potential and comparison of the surface potential [56] Refere Cit d to a fixed reference provides a control signal which UNITED STATES PATENTS can be employed to compensate charge, exposure, 2,956,487 10/1960 Giaimo 355/14 transfer and development- 3,674,353 7/1972 Trachtenberg 355/3 7 Claims, 5 Drawing Figures 1 I f 35%? ca/vr/mL 15 a PATENTEBJAN 29 m:

SHEU 1 BF 2 ELECT/70 METER IMAGE COMPENSATION METHOD AND APPARATUS FOR ELECTROPIIOTOGRAPHIC DEVICES This invention relates to automatic control of electrophotographic devices and more particularly to a method and apparatus for automatic compensation in an electrophotographic reproduction device.

In electrophotography in general, and more specifically in xerography, an electrostatic latent image is formed on an insulating photoreceptor surface, such as, for example, a photoconductive insulating layer or electrophotographic surface by the combined action of an electric field applied through a photoconductive material to cause selective conductivity in accordance with the pattern of actinic or like radiation to which the material is exposed. The result of this combined exposure and field is to form a pattern of electric charge on the photoconductive layer that is known in the art as an electrostatic latent image which is capable of utilization, for example, by deposition being known in the art as development.

The ultimate reproduction quality upon development depends on many factors, and prior art devices have proposed many forms of compensation for such factors. A control signal produced in response to a factor or factors affecting ultimate quality can be typically employed to control charging, exposure, transfer and development in any sequence or combination in order to provide such compensation.

Prior art compensation techniques do not, however, provide a convenient and expedient compensation technique or device which enables photoreceptor characteristics to be adequately determined as a factor in the generation of compensation signals.

In situations of varying temperature characteristics, for example, the intrinsic thermally caused drift in photoreceptor electrical parameters results in a wide variance in image potentials and varying output print quality. Also, variance in parameters between interchangeable photoreceptor materials in actual use, as in drum replacement in a xerographic copier, can also provide a similar unpredictability.

Prior art devices have attempted to measure photoreceptor surface potentials as a means of generating a control signal for monitoring and affecting various factors within the reproduction cycle. However, the use of these devices have not been manifested in a manner appropriate for the compensation of varying photoreceptor parametric characteristics. Further, prior art devices for measuring photoreceptor surface potentials for compensation purposes exhibit unreliability when employed to scan image levels since the image levels themselves are a resultant of photoreceptor parameters. Where photoreceptor surface potentials are measured outside the imaged area, compensatory signals cannot meaningfully be derived since the crucial factor of the photoreceptor response to the activating radia tion is missing.

It is therefore the primary object of the present in vention to provide a novel and unique method and apparatus for compensating for varying photoreceptor parameters.

It is another object of the present invention to provide for measurement of surface potential on a photoreceptor area in a manner which will provide a control signal which may be employed to compensate for varying photoreceptor parameters.

The foregoing objects are accomplished by the detection of photoreceptor surface potential within an area exposed to an image projection but only over that portion of the area which will be exposed to the maximum exposure radiation intensity. Employed in an electrometer probe mounted above the surface of the photoreceptor in juxtaposition with that portion of an exposed area which is external to the image portion of said area. Since the exposure is designed to be wider than the image itself, the image will be bordered on at least one side or edge thereof with sufficient margin to enable an electrometer probe to sense'the surface potential of the fully exposed segment. Measurement of the actual response of the photoreceptor to full impinging radiation provides an accurate indication of the photoreceptor parameters. The developed signal can then be employed in conjunction with a suitable control mechanism to vary accordingly a desired machine operation, such as charging, exposure, transfer or development control.

The foregoing objects and brief description of the present invention, as well as further objects, advantages and features will become more apparent from the following more detailed description and appended drawings wherein FIG. 1 illustrates the present invention in conjunction with a xerographic processor,

FIG. 2 is a detail of the photoreceptor surface,

FIG. 3 is an alternative embodiment of a detail of the photoreceptor surface,

FIG. 4 is a block diagram of an electrometer circuit, and

FIG. 5 is a diagram illustrating a modification of FIG. 4.

An exemplary electrostatic copying apparatus adapted to employ the process of the present invention in the form of a cylindrical drum is illustrated. The drum, generally designated 10 comprising a dielectric material 12 coated on support substrate 14, when in operation, is generally rotated at a uniform velocity in the direction indicated by the arrow so after portions of the drum periphery pass the charging unit 16 and have been uniformly charged they come beneath the energy source 18 for exposing the charged surface to the image to be reproduced. Subsequent to charging and exposure, sections of the drum surface move past the developing unit, generally designated 20. This unit is of the cascade type which includes an outer container or cover 22 with a trough at the bottom containing a supply of developing material. The developing material is picked up from the bottom of the container and cascaded over the drum surface by a number of buckets on an endless driven conveyor belt. This development technique which is more fully described in U.S. Pat. Nos. 2,618,551 and 2,618,552 utilizes a two element developing mixture including finely divided pigmented marking particles or toner and larger carrier beads. The carrier beads serve both to deagglomerate the fine toner particles for easier feeding and charge them by virtue of the relative positions of the toner and carrier material in the triboelectric series. The carrier beads with toner particles clinging to them are cascaded over the drum surface. Electrostatic field from the charge pattern on the drum attracts toner particles from the carrier beads serving to develop the image.

The carrier beads, along with any residual toner particles not utilized during the development step fall back into the bottom of the container 22. The developed image continues around until it comes into contact with the copy web 24 which is passed up against the drum surface by two idler rollers 26 so that the web moves at the same speed as the periphery of the drum. The toner in the developing mixture is periodically replenished from a toner dispenser not shown. A transfer unit 28 is placed behind the web and spaced slightly from it between the rollers 26. This unit is similar in nature to the surface charging mechanism 16 in that both operate on the corona discharge principle. Both the charging device 16 and the transfer unit 28 are connected to a source of high DC potential of the same polarity identified as 30 and 32, respectively, and including corotron discharge wires represented at 34 and 36, respectively, surrounded by a conductive metal shield.

In the case of the corona discharge transfer unit, a charge is deposited on the back of web 24 and this charge is of the same polarity as the charge initially deposited on the drum and also opposite in polarity to the toner particles utilized in developing the latent image. A discharge deposit on the back web 24 pulls the toner particles away from the drum by overcoming the force of attraction between the particles and the charge on the drum. After transfer of the toner image to web 24, the web moves beneath a fixing unit 38 which serves to fuse or permanently fix the toner image to web 24. In this case, a resistance heating-type fixer is illustrated. However, here again, other techniques known in the art may also be utilized including the subjection of the toner image to a solvent vapor or spraying of the toner image with an adhesive film-forming overcoating. After fixing, the web is rewound on a coil 40 for later use.

After passing the transfer station, the drum continues around and moves beneath the cleaning brush 42 which prepares the surface for a new cycle of operation. It should be noted that this apparatus may also be operated at varying speeds by setting the corona discharge unit at a high enough voltage so that the surface of the dielectric will be charged fully at the highest speed.

The foregoing explanation is intended to be exemplary only. The web 24 can be replaced by individual sheets, fed serially into contact with the drum 10. A typical mechanism embodying single sheet feed and illustrative of the mechanical details of the process described above is shown by U.S. Pat. No. 3,301,126 to Osborne et al.

As shown in FIG. 1, the electrometer 44 is positioned with a probe 46 electrostatically coupled to the surface and mounted near the energy source 18, and more particularly adjacent the exposure slit frame 48. The electrical output from the electrometer 44 is a signal representative of the surface potential in the photoreceptor region beneath the electrometer probe. A typical electrometer construction is shown in US. Pat. No. 3,013,203 to Allen et al. The signal is applied through a control unit 50 to control any of the desired machine functions, including charging, exposure, transfer and developing. With respect to developing, the control unit 50 is shown as coupling an output to a develop ment electrode 52 located within the developing unit 20. Typical examples of control of development electrodes for variation of developed images are shown in U.S. Pat. No. 3,424,131 to Aser et al.

Referring to FIG. 2, the development of the electrometer probe 46 is illustrated with reference to the photoreceptor surface in the image area. In FIG. 2, the photoreceptor surface 15 is shown as if seen from the top in the area of the exposure provided by energy source 18. The radiation of the exposure encompasses an area 54 on the surface 15, indicated by the outer dashed line. The actual image is provided over a smaller area 56, illustrated as framcd by a solid line. The marginal area between the inner and outer areas thus defines a portion of the exposed area which will always receive a maximum level of radiation. The electrometer probe 46 is fixed in this marginal area. In this manner, the electrometer probe will always indicate an image potential corresponding to the photoreceptor characteristic or parameters with respect to maximum background levels provided by the radiation source in the image areas. By controlling machine operations in accordance with this level, a more precise control over the quality of the ultimately developed image can be generated without regard to variation in photoreceptor parameters caused by environmental factors or by photoreceptor substitutions. In addition, variations in the output level of the energy source 18 are also compensated in that the quality determination is always based upon the photoreceptors response to exposure. Varying response due either to photoreceptor parameters or to energy source variation will result in the same effect: an image potential level on the photoreceptor surface. By employing the area of maximum exposure as a sampling area, the electrometer output will always provide a corresponding level of reference from which an adjustment or control can be derived.

Referring to FIG. 3, an alternate embodiment is illustrated wherein a separate source of radiation 58 is positioned alongside the primary image source 18. The source 58 provides a separate area 60 in which the electrometer probe 46 is positioned. The intensity of the second source 58 is fixed to correspond to the maximum intensity of the first source 118. Since the photoreceptor surface 15 shares both exposed areas 54 and 60, the response of the photoreceptor to the radiation of the source 58 will be of the same characteristic as the response of the photoreceptor to the maximum intensity radiation of the source 18. Thus, the electrometer probe will be well outside the area of image 56, and that tighter tolerances (or greater image area) between areas 54 and 56 can be provided.

In conjunction with FIGS. 4 and 5, an example of the manner of deriving a control voltage by use of an electrometer and control circuit is set forth.

With reference to FIG. 4, the frame of the electrometer detector probe 46 and its associated electronic circuitry 44 are connected to an adjustable reference potential designated set drum voltage level. The reference potential level is determined by the setting of potentiometer 60 in conjunction with reference voltage source V1.

The detector probe 46 is basically a sensitive electrode. The configuration of the detector probe 46 shown in FIG. 4 is meant to be a generic representation of detector probes of electrostatic modulators that may be exemplarily used according to the invention. These have a modulator device 62 operative in conjunction with sensitive electrode 46 to cause an AC voltage having a magnitude determined by the DC difference potential between the unknown photoreceptor potential and the reference potential to be produced on sensitive electrode 46. The phase of the described AC voltage relative to that of signals supplied by reference oscillator 64 is determined by the DC polarity of this potential difference. The AC voltage produced at sensitive electrode 46 is fed to preamplifier 66.

The operation of the electrostatic modulator generically shown in FIG. 4 is as follows. The detector probe and particularly sensitive electrode 46 is positioned in electrostatic coupling relationship with the photoreceptor surface to produce a detector signal at the sensitive electrode indicative of the magnitude and polarity of the actual surface potential of the drum. Sensitive electrode 46 is exposedto the surface of the drum through a window 16 of the probe housing frame. Reference oscillator 64 produces reference signals at a predetermined frequency which causes modulator device 62 to vary the coupling relationship and thereby produce modulated detector signals having a carrier frequency equal to the predetermined frequency. The modulated detector signals are applied to preamplifier 66.

Reference oscillator 64 is also connected to one input of phase sensitive detector 68 and the output of preamplifier 66 is connected to the other input of phase sensitive detector 68. The latter therefore receives reference signals from oscillator 64 and modulated detector signals, at fixed phase relationship with respect to the reference signals, from preamplifier 66. After demodulation, an output signal indicative of the magni tude and polarity of the actual surface potential differ ence of the surface 15 relative to the present fixed potential is provided at the output of phase sensitive detector 68.

As will be apparent, individual isolation transformers may be interposed between reference oscillator 64 and phase sensitive detector 68 and also between the preamplifier 66 and the phase sensitive detector 68. There would normally be a common power supply for reference oscillator 64 and preamplifier 66 but reference oscillator 64 may operate relative to ground potential with its associated isolation transformer feeding modulator device 62. The set drum voltage level is connected to the frame of the preamplifier and probe, thus establishing the preset fixed potential.

The output of the phase sensitive detector is a DC signal having a magnitude which is linearly related to the AC input thereto. It has a polarity that reverses with phase reversal of the input signal thereto. The output of the phase sensitive detector is connected to integrating circuit 70 which drives integrator controlled high voltage source 72. The latter drives a floating, fixed high voltage supply 74, which may be supplied by the xerography machine being utilized.

The operation of the described system may be illustrated by assuming exemplary values. However, the invention is not limited to such values and a wide range of other values would also be suitable. Where control is of a high voltage corotron, either in development or charging areas, a typical range of corotron supply voltage required to drive a given corotron under an exemplary range of (a) corotron-to-drum spacings, (b) ambient temperature, (c) pressure and (d) relative humidity, while maintaining a constant drum voltage might be 540 to 660 volts. If a minimum integrator controlled high voltage source output of 200 volts is assumed, a fixed high voltage supply source 74 of 5,200 volts should be connected in series with the controlled source 72.

Further, assume a set drum voltage level of 600 volts and an actual drum surface voltage of 590 volts. With the drum voltage being 10 volts less than the set level, a corresponding AC voltage would be produced at sensitive electrode 46 and this would be amplified by preamplifier 66. The output of the preamplifier would be demodulated by phase sensitive detector 68, and the demodulated output thereof would be fed through to integrating circuit 70. The output of the latter would thereby increase and cause a corresponding increase in the output of integrator controlled high voltage source 72 and consequently total corotron supply voltage. The corotron supply voltage applied to the input of the corotron, and thereby the corotron current, would thereby increase.

The increase in corotron current causes a corresponding increase in the drum surface charge density and thus an increase in the drum surface potential.

FIG. 5 shows one type of integrating circuit that may be used. It consists of amplifier 75 connected to function as an'operational amplifier having feedback capacitor 76 and input resistor 77. The capacitance of capacitor 76 must be chosen in consideration of the geometry and speed of rotation of the drum in order to optimize speed'ofqesponse of the system. The system would be unstable and oscillatory if capacitor 76 is too small, and the speed-ofresponse would be excessively slow if capacitor 76 is too large.

The circuit may be modified by connecting capacitor 78 between the input of amplifier 75 and the output corotron supply voltage and reducing the value of capacitor 76 relative to the value thereof in the absence of capacitor 78. The system would then be rendered capable of responding to faster changes in fixed voltage supply 74, assuming integrator controlled high voltage source 72 has a corresponding speed-of-response capability.

By interposing switches S between the output of phase sensitive detector 68 and the input of integrator circuit 70, the integrator input can be restricted to those times when a valid sample is being measured by the electrometer. This provides discontinuous sampling that might be required when plates or sheets, as opposed to drums, are being used in conjunction with the invention.

Thus the invention is not limited to use with a drum, but may also be used in conjunction with plates and sheets. The integrator circuit 70 memorizes the result of the immediate preceding sample, and may thereby serve as an initial condition for correcting subsequent samples.

Fixed high voltage supply 74 can vary slowly without adversely influencing the performance of the system provided (a) the rate-of-change of the fixed voltage is less than the maximum rate-of-change capability of the controller, and (b) the maximum change of the fixed voltage of the drum is within the capability of integrator controlled high voltage source 72 to supply in addition to that previously specified.

The high voltage portion of the circuit of FIGS. 4 and 5 consists of a fixed high voltage supply 74 series connected to integrator controlled high voltage source 72. A single controllable supply, controlled by the low voltage integrator output may besubstituted for elements 72 and 74 if desired for purposes of economy or simplicity of design.

With respect to use of the foregoing techniques in providing specific control of a development electrode, reference is made to the disclosure of US. Pat. No. 3,61 1,982 to S. Coriale et al., which disclosure is incorporated by reference herein.

A While there has been shown and described the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that vari ous changes in form and details of the device may be made by those skilled in the art, without departing from the spirit of the invention.

I claim:

1. A method of producing a control signal for compensating for parametric variation in a photoreceptor surface, the steps of which consist of, charging said surface, exposing an oversized area of said charged surface larger in size that the area normally required for the image to a pattern of radiation, electrostatically detecting the potential level of said oversized area at a point outside the area normally required for the image, comparing said potential level to a fixed potential to derive a control signal, and utilizing said control signal to compensate said photoreceptor surface for parametric variation.

2. In a parametrically compensated electrophotographic reproducing device, the combination comprising a charged photoreceptor surface, means for exposing said charged photoreceptor surface to radiation over an area of said surface, said exposed area including a first portion reserved for the latent image to be reproduced, and a second portion exposed to the same radiation but lying outside said first image portion, means electrostatically coupled to said second portion of said area for measuring the potential level of said second portion of said area, said potential level providing an indication of the parameters of said photoreceptor, and control means coupled to said electrostatically coupled measuring means for supplying a control signal in response to the potential level measured for compensating said reproducing device in accordance with variations in said photoreceptor parameters.

3. The combination of claim 2 wherein said measuring means comprises an electrometer including a probe, said probe electrostatically coupled to said second portion of said area.

4. The combination of claim 2 further including a fixed potential level, said control means being responsive to said fixed level and the potential level measured to provide said control signal. I

5. in a parametrically compensated electrophotographic reproducing device, the combination comprising a charged photoreceptor surface, first means for exposing said charged photoreceptor surface to a first radiation source defining an image over a first area of said surface, second means for exposing a second area of said charged photoreceptor surface to a second source of radiation of a fixed intensity corresponding to the maximum intensity of said first source, means electrostatically coupled to said second area for measuring the potential level of said second area, said potential level providing an indication of the parameters of said photoreceptor and control means coupled to said electrostatically coupled measuring means for supplying a control signal in response to the potential level measured for compensating said reproducing device in accordance with said photoreceptor parameters.

6. The combination of claim 5 wherein said measuring means comprises an electrometer including a probe, said probe electrostatically coupled to said second portion of said area.

7. The combination of claim 5 further including a fixed potential level, said control means being responsive to said fixed level and the potential level measured to provide said control signal. 

1. A method of producing a control signal for compensating for parametric variation in a photoreceptor surface, the steps of which consist of, charging said surface, exposing an oversized area of said charged surface larger in size that the area normally required for the image to a pattern of radiation, electrostatically detecting the potential level of said oversized area at a point outside the area normally required for the image, comparing said potential level to a fixed potential to derive a control signal, and utilizing said control signal to compensate said photoreceptor surface for parametric variation.
 2. In a parametrically compensated electrophotographic reproducing device, the combination comprising a charged photoreceptor surface, means for exposing said charged photoreceptor surface to radiation over an area of said surface, said exposed area including a first portion reserved for the latent image to be reproduced, and a second portion exposed to the same radiation but lying outside said first image portion, means electrostatically coupled to said second portion of said area for measuring the potential level of said second portion of said area, said potential level providing an indication of the parameters of said photoreceptor, and control means coupled to said electrostatically coupled measuring means for supplying a control signal in response to the potential level measured for compensating said reproducing device in accordance with variations in said photoreceptor parameters.
 3. The combination of claim 2 wherein said measuring means comprises an electrometer including a probe, said probe electrostatically coupled to said second portion of said area.
 4. The combination of claim 2 further including a fixed potential level, said control means being responsive to said fixed level and the potential level measured tO provide said control signal.
 5. In a parametrically compensated electrophotographic reproducing device, the combination comprising a charged photoreceptor surface, first means for exposing said charged photoreceptor surface to a first radiation source defining an image over a first area of said surface, second means for exposing a second area of said charged photoreceptor surface to a second source of radiation of a fixed intensity corresponding to the maximum intensity of said first source, means electrostatically coupled to said second area for measuring the potential level of said second area, said potential level providing an indication of the parameters of said photoreceptor and control means coupled to said electrostatically coupled measuring means for supplying a control signal in response to the potential level measured for compensating said reproducing device in accordance with said photoreceptor parameters.
 6. The combination of claim 5 wherein said measuring means comprises an electrometer including a probe, said probe electrostatically coupled to said second portion of said area.
 7. The combination of claim 5 further including a fixed potential level, said control means being responsive to said fixed level and the potential level measured to provide said control signal. 